The present invention relates to space stabilized scanning systems for target tracking, missile guidance and the like. The invention in particular relates to a method and system for scanning an area of interest with a sensor that is referenced to a space stabilized reference member such as a gyroscope momentum wheel or member.
In weapons system or other systems that require the locating and/or tracking of an object such as a target, a sensor or detector is usually employed to sense energy (e.g. light, heat, radio frequency radiation, etc) received either directly or by reflection from the object. The sensor is typically arranged to produce an electrical signal related to the magnitude of the energy received from a particular direction. In this manner, a sensor line-of-sight is established, and movement of this line-of-sight can be controlled for purposes such as acquiring and tracking targets. When tracking an object, for example, the system can produce, and use for position control purposes, an error signal representing the amount and direction of difference between the sensor line-of-sight and the object line-of-sight (i.e. the line-of-sight between the sensor and the object being tracked).
In order to produce error signals that are useful for tracking and guidance in systems such as missile guidance system where the structural frame of reference is constantly moving, a space stabilized reference for the sensor line-of-sight is usually provided. The space stabilized reference is often provided in such systems by a gyroscope which includes a spinning mass. The spinning mass is referred to herein as a momentum wheel or member and is referred to as part of the gyroscope although it should be understood that it is not uncommon to refer to the spinning mass itself as a gyroscope. The momentum member is typically supported on a multi-degree of freedom gimbal system such that it provides a space stabilized platform to which the sensor or detector line-of-sight may be referenced.
By way of specific example, a sensor may be mounted in such a system so as to provide signals related to the amount of electromagnetic wave energy (e.g. infrared, laser visual, radio frequency, etc.) received by the sensor from a direction having a predetermined orientation with respect to the spin axis of the gyroscope. The sensor may be mounted on the gyroscope in selective alignment with the spin axis to accomplish this purpose, or an optical system mounted on the gyroscope may be arranged to direct, to the sensor, energy received along the spin axis. In either arrangement, the sensor line-of-sight is somehow referenced to the gyroscope spin axis and the magnitude of the signal produced by the sensor will be related to the position of the spin axis of the gyroscope relative to the object from which the wave energy is received. Thus, there is a space stabilized reference for the sensor.
With any multi-degree of freedom spinning mass there is an inherent instability that causes the mass to nutate, i.e. causes the spin axis to revolve or "wobble" about a fixed spatial axis. In a tracking system of the type previously used to generate guidance commands for a missile, such nutation has been considered undesirable, and mechanical dampers are typically provided to introduce a dampening effect on the natural nutation. It will be appreciated that if the spinning mass nutates in an uncontrolled manner, the stabilized space reference for the sensor line-of-sight is lost and accurate tracking or other control functions cannot be maintained.
In various modes of system operation, e.g. searching, acquisition or tracking, certain controlled movement of the sensor line-of-sight may be desirable or even required. It has therefore been common to maintain the gyroscope spinning mass in a stabilized position with the aforementioned mechanical dampers and to move the optical system or the sensor relative to the gyroscope spin axis in some predetermined manner in order to obtain a desired pattern of sensor line-of-sight movement, e.g. a scanning pattern for target acquisition or tracking.
If the scanning pattern must be varied in a precisely controlled manner then this method of obtaining a desired scanning pattern requires complex mechanisms and may introduce inaccuracies because of the required relative movement (and resultant mechanical coupling) between these mechanism. Complexity, of course, leads not only to increased cost and decreased reliability, but more importantly, it can also result in increased size and weight, two factors that are extremely important in weapons systems.
It is thus an object of the present invention to provide a novel method and a space stabilized scanning system that overcomes the foregoing and other problems associated with prior art system.
Another object of the present invention is to provide a stable system for inducing and controlling gyroscopic nutation to serve a useful purpose. A novel method and system has been devised as a part of the invention for excitation and control of gyro-mechanical instability. A fixed amplitude excitation at the natural nutation frequency of a spinning mass is introduced to cause nutation in the pitch and yaw axes. In the absence of a negative rate feedback signal maximum circular nutation, with rates measured in thousands of degrees per second, is obtained. Nutation control is obtained by comparing the amplitudes of the rate feedback and nutation command signals and apply a torque to reduce the difference to zero. Differential variation in the amplitude of the nutation command signals is used to control the eccentricity and pattern of nutation achieved, thereby permitting generation of not only circular or elliptical patterns, but also more complex patterns such as rosettes, etc. The gyroscopic excitation and control mechanism according to the invention may be electrical, electronic mechanical, pneumatic, hydraulic, fluid or any combination of the foregoing implementations.
Yet another object of the present invention is to devise a means of controlling the frequency of nutation excitation of a momentum wheel to maintain correspondence with the natural resonant frequency of momentum wheel nutation which continuously varies as the spin rate of the wheel changes. To minimize torque excitation requirements and therefore power input requirements, the ratio of the nutation excitation frequency to the natural nutation frequency must be maintained within sharp null limits very close to one. A difficulty encountered in maintaining a one to one frequency ratio is that the natural nutation frequency varies in direct proportion to changes in spin rate.
A novel method and system for maintaining a one-to-one frequency relationship between excitation torque and the natural resonance is embodied according to one aspect of the invention in a control system which maintains a fixed phase relationship between a gimbal axis nutation component obtained as the output of one or both of the pitch or yaw position sensors. A zero or very small phase angle between the position sensor output signal and the excitation torque signal is maintained by increasing or decreasing the torque excitation frequency as commanded by a closed loop system to minimize the relative phase difference between the two signals.
The present invention has, as one of its further objects, the provision of a novel method and system for scanning an area of interest with a sensor through an electronically controlled nutation of a space stabilized momentum member such as the rotor of a gyroscope while retaining the low frequency space stabilization characteristics of the member.
Yet another object of the present invention is the provision of a novel method and system for scanning an area of interest relative to a space stabilized reference member wherein a natural tendency of the stabilized reference member to nutate is utilized advantageously to cause a controlled nutation of the stabilized reference member.
Still another object of the present invention is to provide a novel method and system for scanning an area of interest relative to a space stabilized reference member wherein mechanical nutation dampers are removed from the reference members, a sensor line-of-sight is fixed relative to the reference member and the reference member is caused to nutate in a controlled manner in order to cause the sensor line-of-sight to move in a controlled manner through the area of interest proximate to the target.
An object of the present invention is also to cause a gyro spin axis, through controlled nutation, to cross and re-cross a particular point in space, identifiable to the selected sensor device utilized in the particular application. This high frequency repetitive crossing of the identifiable point may be in any number of geometric patterns such as circles, rosettes, or other scan pattern devised to optimize the sensor response and provide an error signal which is fed back to position the momentum wheel to center the patterns over the selected points to achieve tracking of that point.
These and other objects and advantages of the present invention will become apparent to one skilled in the art to which the invention pertains from the following detailed description when read in conjunction with the appended drawings.